Genetic and biochemical studies with enterobacteria reveal that nitrogen control in these organisms in quite complex. Nitrogen control can be defined as the adjustment in enzyme synthesis made by cells in response to the availability of nitrogen in the growth medium. The levels of the ammonia-assimilatory enzymes, as well as some amino acid transport systems and catabolic enzymes are affected by this process. The function of the several genetically identifiable regulatory elements which participate in this control system has yet to be determined. In this laboratory, studies have focused on the possibility that an additional regulatory role in nitrogen control might exist for some of the proteins which participate in the cascade for modulation of the enzyme activity of glutamine synthetase (GS). The evidence so far indicates an interaction between the elements of the activity modulation cascade and the genetic elements involved in the regulation of synthesis.